Move "You can import another Fastfile by using the import action" to better place

https://docs.fastlane.tools/actions/ has

> You can import another Fastfile by using the import action. This is useful if you have shared lanes across multiple apps and you want to store a Fastfile in a separate folder. The path must be relative to the Fastfile this is called from.
> `import './path/to/other/Fastfile'`

This is kind of misplaced here, so it should moved to a better place.